The demand for either higher output power from several smaller off-the-shelf power supplies or their fault-tolerant redundant operation in the case of critical loads, resulted in almost mandatory feature of switched mode power supplies: paralleling with current sharing. The parallel operation means that two or more power supplies are connected to the common source of power and deliver the power to the common load. When parallel operation of switching converter is mentioned, almost invariably the most common first question asked is: how is the load current shared between two converters? Clearly, equal sharing of the total current would be most desirable. Indeed, this is made possible and practical with the advent of the prior-art feedback control and switch protection method called current mode programming. However, a more fundamental second question, which should come even before current sharing question, would be: could the two converters connected in parallel even start properly and go gracefully through the soft-start mechanism without causing any performance problems. Worse yet, could the parallel operation during the start-up result in catastrophic failure of either one or both converters? The answer to that question is affirmative unless measures to prevent that from taking place are undertaken, such as disclosed in this invention.
This invention reveals the sources of such potential problems during the start-up of converter parallel operation and discloses a new method, termed Voltage Sense method, how to prevent such problems during the start-up. A number of alternative circuit implementations of this Voltage Sense method are presented which eliminate those problems during the start-up. The switching converters with diode rectifiers on the output do not experience such start-up problems. However, in majority of the applications for powering modern electronics equipment, the low voltage power supplies are needed with recent 5V voltage standards being replaced with 3.3V, 2.5V and even 1.8V to power modern microprocessors, which require ultra low voltages and very high currents to operate. The switching converters delivering such low output voltages invariably operate with the output rectifier diodes replaced with the synchronous rectifiers, which drastically reduce the voltage drops of the diode switch (typically 0.7V to 1V) to 0.05V or lower voltage drop across MOSFET synchronous rectifier switch, thus dramatically reducing the conduction losses and improving the efficiency of converter operation. The synchronous rectifiers also make each converter current bi-directional, thus allowing the power flow to be also bi-directional. This, in turn, is precisely one of the reasons why the converters with synchronous rectifiers experience series problems, which result in failures when their parallel operation is attempting without recognizing the source of the problems and without use of special circuit preventive measures.
In the past, the prior-art methods and circuit preventive measures have focused on two methods and circuit implementations, termed here Fixed Timing and Current Sense methods, which mitigated only some start-up problems with synchronous rectifiers and even than only to a certain degree but not completely. The third method, termed here Voltage Sense method, of the present invention, solved all these problems as described in more details in later section. All three methods are based on disabling the synchronous rectifiers during the initial start-up, but differ in how the decision to enable synchronous rectifiers is made:                1. Fixed Timing method and circuit implementation enables synchronous rectifiers after a fixed time interval has elapsed from the beginning of the converter operation;        2. Current Sense method and circuit implementation enables synchronous rectifiers based on sensing the DC current delivered by each converter;        3. Voltage Sense method and circuit implementation enables synchronous rectifiers based on sensing the simulated voltage to determine the optimum time to enable synchronous rectifiers.        
There are a number of prior-art circuits, which address the start-up problem of parallel converters with output synchronous rectifiers. However, they are all based on various implementations of the Current Sense method in which converter's DC output current is sensed and used to determine the instant at which synchronous rectifiers should be enabled. Three such prior-art patents are:    1. U.S. Pat. No. 5,636,116 by Boylan, et al;    2. U.S. Pat. No. 5,663,877 by Dittly, et al;    3. U.S. Pat. No. 6,038,154 by Milavec, et al.
Based on a number of synchronous rectifiers on the output side, the switching converters could also be divided into following two categories:                1. Switching converters with Single Synchronous Rectifier on secondary side.        2. Switching converters with Two Synchronous Rectifiers on secondary side.        
It is demonstrated that Fixed Timing control and Current Sense method and circuit implementations address start-up problems partially and only in converters with single synchronous rectifiers. In addition, some serious problems of excessive voltage stress on input side switching devices for low voltage converters remain. Voltage Sense method, however, is suitable for all converters regardless of number of synchronous rectifiers in the output and eliminates high voltage stress of input switches for the converters with ultra low output voltages such as 1.8V.
Definitions
The following notation is consistently used throughout this text in order to facilitate easier delineation between various quantities:                1. I1, V2—The customary notation is to use capital letters, such as I1 and V2 to designate quantities constant in time, such as DC current in converters or DC voltages. However, herein, during the start-up of the switching converters these quantities are also changing in time, such as for example, the output DC voltage V is increasing gradually from zero volts to its final value, the regulated output voltage. Thus, the DC quantities are for purposes of this disclosure also assumed to be function of time during the start-up transient        2. S1, S2, S′1, S′2—Switch designations respectively for input switch, output switch, complementary input switch, and complementary output switch;        3. D—The duty ratio is defined as D=tON/TS where tON is the ON time interval during which the input switch is closed (turned ON) and TS is the switching period defined as TS=1/fS where fS is a switching frequency;        4. D′—The complementary duty ratio D′ is defined as D′=tOFF/TS where tOFF is the OFF time interval during which the input switch S1 is open (turned OFF) and the complementary switch S′1 is closed.Distinction Between Current Bi-Directional and Synchronous Rectifier Switches        
It is also important to highlight at the very beginning distinction between Current Bi-directional Switches (CBS) and Synchronous Rectifier Switches. Current Bi-directional Switch (CBS) is a three-terminal, controllable semiconductor switching device, which can conduct the current in either direction between two terminals, when the appropriate control signal is given at the third terminal to turn-ON device. This CBS switch also blocks the voltage of only one polarity between the said two terminals when the control signal is given to the third terminal to turn-OFF device. A good example of such a CBS switch is a power MOSFET transistor, which has also a parasitic body-diode. This body-diode effectively introduces the limitation of voltage blocking capability to one polarity only.
In many applications designed for low output voltages, output diodes are replaced by MOSFET devices to by-pass conduction from the body-diode into a MOSFET channel of a respective device. This special application of MOSFETs is called synchronous rectification to signify the fact that MOSFETS are conducting during exactly the same intervals that their respective body-diodes would have conducted if used alone (CBS switch drive disabled) but in a response to other duty ratio controlled switches in the converter and other converter circuit conditions. Thus, synchronous rectifiers represent a rather limited application of CBS switches, whose timing control is not independent but limited to the conduction times of body-diodes.
CBS switches on the other hand do not have such timing limitations. They are completely independently controlled and can be, for example, turned ON before the internal body-diode would have started conduction in response to converter circuit conditions. The switching converters which use such performance of CBS switches for output rectification are started now to appear, since they bring additional performance features, such as lossless switching and further improved efficiency. Nevertheless, both output CBS output switches, and their limited application as synchronous rectifiers, result in the same problems during the start-up of parallel converters using them. Thus, the Voltage Sense method and its Voltage Sense circuit implementations of the present invention will be equally applicable to both categories of the converters.